The trend amongst designers these days is towards higher integration of control circuitry and power output devices on the same chip. Such integration calls for adequate circuit protection functions in order to guarantee high circuit reliability in extreme conditions.
For output power devices such as power transistors, the temperature protection function is an important function, particularly under short circuit conditions. The power transistors may be in bipolar and/or MOS technology. The temperature protection function senses the temperature of the heat source in the power transistors, which is the active junction or channel of the power transistor, and in response to the sensed temperature controls the drive to the power transistor.
A standard technique which is used to sense the `junction` temperature of a power transistor consists of a temperature sensor, such as a diode and/or a transistor, placed in the vicinity of the power transistor. However, the accuracy of the temperature sensing of this technique is low due to the fact that silicon is a poor heat conductor and that the temperature sensor cannot be located close enough to the active junction or channel of the power transistor.
Another disadvantage with this technique is that with circuits having a low thermal resistance from the power package to an `infinite` heatsink, this technique is inefficient and can lead to the melt down of the power transistor. The operating conditions for which melt down may occur is often found in automotive applications where heat sink temperature varies between -40.degree. C. and 125.degree. C.
An improvement on the above technique is disclosed in U.S. Pat. No. 4,903,106. This patent discloses integrating a temperature sensing device on the same substrate as a power device. This technique avoids the need for a temperature sensor exterior to the power device. Since the temperature sensing device is formed on the same substrate with the power device, the accuracy of the sensed temperature is improved.
However, the structure of the temperature sensing device disclosed in this U.S. patent is such that parasitic components of the device cannot be neglected during normal operation. In fact, these parasitic components can affect the temperature sensing device to the extent that it provides wrong information to a temperature detector.
Furthermore, although the temperature sensing device and power device have a common substrate, they do not share common junctions and require substrate isolation. Thus, the temperature sensing device is not electrically merged with the power device, and so the physical distance between the power device and the temperature sensing device is still sufficiently large to limit the accuracy of temperature sensing.